1. Field
This disclosure relates generally to circuitry useable with a touch panel, and more specifically, to touch panel detection circuitry.
2. Related Art
Touch panels are commonly used as input devices in a variety of applications. For example, circuitry can sense when an object like a pen applies a pressure to a touch panel, forcing the two faces of the touch panel to touch. Typically, when no pressure is applied to a touch panel, the touch panel capacitor charges up through a current limiting resistor, and when a pressure is applied, the touch panel discharges to indicate that pressure has been applied to the touch panel (i.e. to indicate a pen down event). At this point, further processing can occur to determine, for example, if subsequent pressure has been applied or the position of the applied pressure. However, as technology advances, touch panels continue to increase in size (panel area) and decrease in thickness, resulting in higher capacitance values. Since the touch panel capacitor charges up through the current limiting resistor, these higher capacitance values result in higher resistance-capacitance (RC) time constants which can detrimentally slow the charging of the touch panel capacitor resulting in false indications of applied pressure (i.e. false pen down events). These false indications reduce reliability of touch panel display systems.
One solution available today attempts to reduce the RC time constant by reducing the resistance value of the current limiting resistor described above. However, in this solution, power consumption increases during a pen down event due to the increased current flow through the smaller current limiting resistor. This can be especially problematic in hand-held or portable touch panel display systems where power consumption should be minimized in order to extend battery life. Another solution available today eliminates the current limiting resistor all together. This solution applies a periodic pulse to periodically charge the touch panel capacitor, where between charge pulses, the touch panel capacitor is left floating. However, this solution is more susceptible to noise, thus resulting in reduced reliability, and also causes increased electro-magnetic interference (EMI).